Di-alkyl compounds of Group 2b metals find increasing use in the electronics industry. Compounds and alloys containing elements such as zinc and cadmium are deposited on substrates from volatile precursor compounds, such as their respective di-alkyl compounds, by thermal decomposition from the vapor phase to give a thin (semiconductor) layer. This technique is known in the industry as Metal Organic Chemical Vapor Deposition (MOCVD). When an epitaxial layer is grown the technique is better known as Metal Organic Vapor Phase Epitaxy (MOVPE). A process for the deposition of zinc sulfide films on a semiconductor substrate in which di-alkyl zinc is used in combination with hydrogen sulfide is described in European patent application No. 405,875. A method for the preparation of epitaxial layers of zinc sulfide and zinc selenide is described in U.K. patent application No. 2,221,924.
The presence of impurities in such semiconductor layers has a substantial adverse effect on both their electrical and their optical properties. It is desired that the precursor compounds, such as the di-alkyl compounds of the Group 2b metals, are very pure. For the production of p-type zinc selenide layers for use in opto-electronic devices, the iodine content in the zinc precursor is of the utmost importance. Iodine is an n-type dopant and hence, controlled p-type doping can only be achieved if the iodine content in the epitaxial layer, and therefore in the zinc and selenium precursors, is very low, preferably below 1 ppm (by weight). Hence, it is not surprising that efforts have been made to purify these precursors, and particularly the di-alkyl compounds.
In U.S. patent specification No. 4,812,586 the purification of impure dimethylcadmium and dimethylzinc is described. According to this specification impure di-alkyl compounds are prepared by reacting methyl halide with magnesium to yield a Grignard reagent. This reagent is subsequently reacted with cadmium halide or zinc halide to yield the respective impure di-alkyl metal compound. This compound is purified by adduct formation with specific amino compounds followed by removal of impurities from the adduct. The purified adduct is subsequently dissociated and subjected to distillation to yield the purified di-alkyl compound. This process requires several steps.
In U.K. patent specification No. 1,242,789 a process is described in which metallic zinc is alkylated using alkyl halides in the presence of other metals, i.e., alkali metals. According to the examples of this specification the yields obtained in these examples range from 53% to 85%. However, it is apparent that in the example giving the highest yield, 0.220 mole zinc yielded 5.8 g or 47 mmole diethylzinc. Hence, the yield based on total zinc consumption was only 21%. This process requires an excess of zinc in addition to that which is contained in the di-alkyl compound, because zinc is used as halogen acceptor. If the use of very pure zinc is required, as would be the case for MOCVD or MOVPE purposes, this process would involve the waste of considerable amounts of pure zinc.